Okay so before I say anything, I'm going to apologise because I'm not at home this week and therefore I don't have access to the sources I would normally reference to answer something like this. So at the moment I'm not going to provide many sources. I wasn't going to reply but I notice it's been half a day and you haven't gotten a reply so something is better than anything.

So, with any historical question, the answer is almost never "x" or "y" but rather, as you put it, "a combination of factors". The "double envelopment" at Cannae might be one of the most oversimplified and therefore exaggerated explanations for a great military victory in history. The legacy of Hannibal's double envelopment, completely destroying a huge Roman army for possibly one of the most comprehensive one-day tactical victories in military history, has been such that even the German army in WWI and WWII, renowned for its intellectual culture and excellent staff work, were trying to replicate it on a much larger scale with their invasions of France. Hannibal's "double envelopment" has ever since 216 BC been the gold standard for tactical generalship, and yet it really does ignore the real reasons for Hannibal's victory and reduce it to some cool looking lines on endlessly replicated tactical diagrams.

The "double envelopment" was, basically, an extremely simple tactic that not only did Hannibal not invent, but was something that almost every half decent ancient general attempted to do as often as possible. Usually only a single envelopment was possible, sometimes a double envelopment could be achieved with significant infantry and/or cavalry superiority, but it really isn't that special. Alexander the Great won all of his 3 major battles with strong flank attacks with cavalry, the Spartans used to wheel off small detachments from their phalanxes to flank enemy phalanxes, ever since we invented tank formations every offensive operation has tried to achieve envelopment, the examples are endless. A compete double envelopment, however, was almost impossible to achieve. This is because it took numerical superiority against a sturdy enemy that would not immediately panic when partially flanked (like the Romans), it took cavalry superiority (the most important flanking unit, the one that traps the rear), and it took a highly aggressive and solid infantry core. When you're dealing with the Romans on the open field with two consular armies, only Hannibal had these things. Hannibal had the infantry core that was good enough it could fix Roman infantry in place, and give ground in the centre (all part of his plan) without totally breaking, an extremely difficult manoeuvre. Hannibal had cavalry superiority, and he had serious numbers.

To put it simply however, the key point of difference between Cannae and Dertosa was that the Carthaginians lost the cavalry battle. That's the most important factor. Hannibal's actual movements at Cannae were not that complicated, he gets too much credit for that. All he did was create a concave infantry formation that fixed the Romans and confused their formation so that a cavalry charge could strike them in the rear and cause decisive chaos. It's pretty simple, it's just not easy. How hard is it? Well, Hannibal is the only guy who ever managed it against the Romans. It takes a long time for cavalry to win a cavalry battle like they did at Cannae, pursue their opposing cavalry off the field, then ride several kilometres around to the rear of an army and hit them where it hurts. It probably took an hour at a minimum. In this time, Hannibal's infantry needed to absorb the shock of a larger number of more heavily equipped infantry that were highly aggressive and motivated. He calculated that he could do that, and he got it right, but if he'd gotten it wrong his entire formation would have shattered and the Romans would have organised their formations to meet the cavalry charge and the Second Punic War would have pretty much ended right there. Hannibal won the cavalry battle, Hasdrubal didn't. Without the cavalry to come and hit the Romans, even the solid Carthaginian infantry weren't going to push a larger number of Romans off the field in a straight up fight, and the double envelopment became impossible.

Notice that Scipio returned the favour to Hannibal at Zama. He won the cavalry battle, and the cavalry hit the Carthaginian formation in the rear, even though Hannibal's veterans were holding well against Scipio's principes.

So the "double envelopment" is a great, fantastic stratagem if it can be pulled off. That nobody really managed it on the scale of Hannibal speaks volumes for his judgement as a commander and the overconfidence of the Romans. But the most important thing to remember is that if Hannibal had lost the cavalry battle at Cannae (unlikely considering his superior cavalry numbers, compare the reversal of this numbers situation at Zama), he probably wouldn't have won the battle. Hasdrubal lost the cavalry battle at Dertosa, and so he lost the capacity to complete a full envelopment in the Hannibalic style, which required a rear charge by cavalry.

Long story short, it was a successful cavalry engagement, solid Carthaginian infantry line, Roman overconfidence and sub-optimal force composition, and his own impeccable judgement that allowed Hannibal to achieve what nobody else could. I hope this has answered your question.

Note on sources:

I'm sorry I didn't provide sources at this time. I can provide some later if necessary. Probably my only suggestion at the moment is to compare the accounts of Cannae (Book 3) and Zama (Book 15) in Polybius. I do have a copy of Adrian Goldsworthy's "The Fall of Carthage" on me, and he provides a brief summary of the battle on page 214. Goldsworthy emphasises more than I have the powerful effect of the heavy Libyan spearmen (perhaps Hannibal's best troops) closing on the Roman flanks when Hannibal's centre gave way, but also accepts that if Hannibal's centre had given way earlier, or the cavalry battle had gone sideways, then the numerically superior and heavily equipped Roman infantry could probably still have fought their way out. They certainly could not have been trapped and destroyed enmasse without the cavalry.